Featured Image for Seafood Poisoning - Marine Toxins and What You Need to Know – Part 1: Shellfish

We love our fish and shellfish in America.  Entire towns are built around the industries and fish and shellfish routinely appear on our plates at home and are increasingly popular choices on restaurant menus.  But as you are probably aware, fish bring their own set of food safety and food risk problems.  Marine toxins are the usual cause of seafood risk, and in this two-part series, we will look at the toxins that occur in shellfish (Part 1) and then the toxins that occur in fin fish (Part 2).

According to the National Oceanographic and Atmospheric Administration (NOAA), 91 percent of the seafood that Americans consume is imported and a lot of that is shellfish.  Shellfish is a very broad term that includes mollusks like clams, mussels, scallops, oysters, and cockles, as well as crustaceans like crab, shrimp, crayfish and lobsters. Shrimp is the most popular seafood in America, and clams come in at number 10 in popularity.

How does it happen?

Marine toxins found in shellfish are typically the result of the ingestion of one of a number of different types of algae. The algae and the toxins exist in low concentrations at all times. However, during warmer months (June to October in the northern hemisphere) a phenomenon called a Harmful Algal Bloom (HAB) can occur.  Better known as a red tide, an HAB dramatically increases the amount of algal toxins present. HABs can cover very large areas, sometimes as large as 1,000 square miles in size.

Mollusks are the most likely shellfish to become toxic. They are filter feeders living off of the algae in their environment and can accumulate very high levels of toxins during harmful algal blooms. Shellfish can eventually pass the toxins through their system, but that can take anywhere from days to months after the bloom has subsided. 

These toxins can only be detected in shellfish through laboratory testing. The shellfish will not smell, look, or taste spoiled and the toxins cannot be destroyed by cooking, boiling, or freezing, making them extremely dangerous to humans. The Centers for Disease Control and Prevention (CDC) estimates that although thousands die worldwide, there are only about 30 cases per year occur in the US from shellfish poisoning. The number of those suffering unreported illness is likely much higher.

Four types of shellfish poisoning – causes and symptoms

Amnesic Shellfish Poisoning (ASP)

Amnesic shellfish poisoning (ASP) is a fairly rare type of shellfish poisoning caused by a biotoxin called domoic acid. This toxin is produced by a species of algae called Pseudo-nitzschia. Consuming shellfish contaminated with domoic acid can make you ill. Symptoms of ASP include vomiting, nausea, diarrhea, and abdominal cramps, all within 24 hours of consuming the shellfish.

If the poisoning is severe, neurological symptoms can appear within 48 hours and can include headache, dizziness, confusion, disorientation, loss of short-term memory (which can be permanent), motor weakness, seizures, respiratory secretions, cardiac arrhythmias, coma, and possible death.

The types of shellfish most often affected are razor clams, mussels, clams, and oysters. Other types of seafood may also become contaminated because they feed on shellfish. Shellfish from coastal areas are more likely to be affected because this is where algae are more likely to be found. Areas where ASP is commonly found are Maine, Massachusetts, California, Washington, and Oregon.

Diarrhetic Shellfish Poisoning (DSP)

Diarrhetic Shellfish Poisoning (DSP) is caused by a biotoxin called okadaic acid that is produced by a species of algae called Dinophysis. Consuming shellfish contaminated with okadaic acid can make you mildly ill.  Symptoms include diarrhea, nausea, abdominal cramps, vomiting, and headache. Symptoms can appear anywhere from 30 minutes to 15 hours after consuming the affected product and usually last around three days. No fatalities have ever been reported in connection with DSP.

The toxins that cause DSP have been commonly found throughout European countries and the Western Coast of Canada. Most recently the toxin has been found off the coast of Washington state and is believed to be responsible for several illnesses. The types of shellfish commonly found with okadaic acid in their systems are mussels, clams, and scallops.

Neurotoxic Shellfish Poisoning (NSP)

Neurotoxic shellfish poisoning (NSP) is caused by a group of more than 10 toxins called brevetoxins, which are produced by a species of algae called Karenia brevis. Algal blooms containing these toxins occur frequently in the Gulf of Mexico. Few cases of NSP are reported annually in the United States and are usually associated with recreationally harvested shellfish during or right after an algal bloom has occurred.

Gastrointestinal and neurological symptoms occur when contaminated shellfish are ingested. Symptoms can include nausea, vomiting, paresthesias (a burning, prickling, or tingling sensation) of the mouth, lips, and tongue, distal paresthesias, ataxia (lack of muscle coordination), slurred speech, and dizziness. More severe neurological symptoms can occur and include partial paralysis and respiratory distress. No fatalities have ever been recorded, but hospitalizations do occur.

Brevetoxins can be commonly found in clams, oysters, whelks, mussels, conch, and coquinas.

Paralytic Shellfish Poisoning (PSP)

Paralytic shellfish poisoning (PSP) is caused by a group of toxins, the most potent being saxitoxin, which is a neurotoxin produced naturally by numerous species of algae.  All mollusks including clams, oysters, mussels, and scallops can accumulate PSP toxins in their system.

Because crabs feed on shellfish, there is the possibility that certain parts of the crab may accumulate the toxins. It is believed that crabmeat will not accumulate the toxin, but the guts may contain high levels of the toxins. It is advised to clean the crab thoroughly, discarding the guts. PSP toxins are commonly found on the Northern Atlantic and Pacific coasts of North America.

If contaminated shellfish are ingested, symptoms of PSP can develop within 15 minutes and usually with two hours. Early symptoms include tingling of the lips and tongue. If large amounts of the toxins were ingested, the tingling may move to the fingers and toes, then loss of control of arms and legs, followed by difficulty breathing. In severe cases, muscles of the chest and abdomen become paralyzed. Death can occur in as little as two hours if the muscles used for breathing are paralyzed.

Protecting yourself

So what steps can you take to help protect your health and the health of your family? As previously mentioned, these toxins cannot be destroyed by cooking, boiling, or freezing and cannot be detected by sight, taste, or smell.

  • Always buy your shellfish from a reputable source. Commercially harvested shellfish undergo periodic testing to detect the presence of any toxins. Commercial shellfish beds are also continuously monitored for the increasing presence of algae. If an algal bloom is suspected, the area will be closed for harvesting and will remain closed until the danger has passed.
  • Recreational shellfish harvesting is behind most cases of shellfish poisoning. If you are going to participate in recreational shellfish harvesting, check with local health authorities to ensure the area you will be in is not affected by or has not been recently been affected by an algal bloom. Most coastal states have a Department of Health webpage where the information can be found. Here is one from Washington State. Shellfish can eventually pass the toxins through their system, but it can often take months for them to do so.
  • Do not purchase or consume shellfish that is intended to be used as bait. They are not required to meet the same health standards as shellfish meant for human consumption, leaving open the possibility they could contain high levels of marine toxins or bacteria that could make individuals very ill.
  • Be aware that shellfish poisoning is much more likely in the summer than in the winter because the toxic algae thrive in warmer water.
  • Ice seafood or keep it refrigerated at 38°F or below.
  • If you are pregnant: Your immune system is depressed; this is a normal, naturally occurring event during pregnancy.  Shellfish, particularly raw shellfish, are routinely contaminated with certain types of bacteria, particularly Vibrio vulnificus and Vibrio haemolyticus. The Mayo Clinic recommends that you do not consume any raw or undercooked shellfish during your pregnancy.  If you become infected, your symptoms are likely to be more serious.  It has not been demonstrated that these bacteria cross the placental barrier, but similar bacteria do. If you love oysters, just cook them thoroughly.  This will kill the bacteria, but remember that the shellfish toxins described here are unaffected by cooking.

If you get sick

There is not much in the way of specific treatment for any of these illnesses, and identification of the specific type of seafood poisoning is not required for palliative treatment.  However, if reactions are severe, emergency treatment should be sought. If you are vomiting or have diarrhea, be careful not to become dehydrated, and maintain a good electrolyte balance. Doctors recommend that you stick to your normal diet.

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Now that you know about the various types of shellfish poisoning, go to Harmful Algae and check out their maps showing the distribution of the various shellfish poisoning events over the past ten years in the United States. This is an excellent resource to help you better assess the risk in your area, or in areas where your seafood comes from.

Stay tuned for Part 2 of Marine Toxins, where we will discuss two types of illnesses caused by marine toxins in finfish: Scombrotoxic fish poisoning and Ciguatera poisoning.